This invention relates to the field of semiconductor light sources, specifically light sources that produce incoherent and coherent optical emission (lasers) using current filaments in semiconductors as the source of the optical emission.
Miniature short pulse lasers are used in various applications including: active optical sensing and imaging in clear and limited visibility environments; fiber optic isolation and communication; scientific exploration involving optical excitation and probing techniques; optical reading and writing of digital information for computer access; direct and indirect optical ignition of fuels and explosives; medical procedures to illuminate, heat, burn, or ablate tissue; and micro-machining micron-size features patterned over large areas of metals and alloys. These applications generally require short pulses and low beam divergence. Conventional semiconductor lasers are unsuitable for many of these applications because they do not deliver sufficient high beam quality optical energy.
Conventional semiconductor lasers can be placed in three general categories: injection-pumped, optically-pumped, and electron-beam-pumped. A conventional injection-pumped or diode laser is pumped with carriers injected across a diode junction. There are many types of injection lasers: single and double heterojunction, quantum well, and vertical cavity surface emitting (VCSEL), for example. See, e.g., Matthews et al., U.S. Pat. No. 4,350,960 (1982); Goodwin et al., U.S. Pat. No. 4,631,729 (1986); Morrison et al., U.S. Pat. No. 4,633,477 (1986); Taneya et al., U.S. Pat. No. 4,730,326 (1988); Taneya et al., U.S. Pat. No. 4,791,651 (1988); Taneya et al., U.S. Pat. No. 4,870,651 (1989). All of these lasers are made with n-type and p-type regions which form junctions. They are limited in one dimension to the effective length of a junction, which is typically 1-2 microns. Injection-pumped lasers make up most of the commercial semiconductor lasers available today. They are very efficient and can be fabricated monolithically in large arrays. However, most arrays are not spatially coherent and therefore not as useful for many applications as a single large laser with the same total energy per pulse or peak power. Flared waveguide lasers and master oscillator power amplifiers are other examples of large area single-element injection lasers, that have coherent, diffraction-limited, output beams.
A second type of semiconductor laser is one which is pumped optically. New types of materials or lasing configurations are often tested first by pumping the device optically instead of injecting current. Electrical contacts are not necessary, so this technique is very useful in diagnosing new structures. However optical pumping can require another laser to do the pumping, which makes it not very efficient. This type of laser can be much larger, but when an optically pumped laser is the goal, materials other than semiconductors are generally used as the lasing medium. These are known as xe2x80x9csolid statexe2x80x9d lasers, such as Nd:YAG or Ti:Al2O3 (Titanium-Saphire). These materials can store the energy longer in a different lasing scheme than semiconductors.
A third type of semiconductor laser is pumped with a high energy electron beam. See Bogdankevich, xe2x80x9cElectron-beam-pumped semiconductor lasersxe2x80x9d, Quantum Electronics 24(12) 1031-1050 (1994). The electrons excite many xe2x80x9chotxe2x80x9d carriers across the semiconductor band gap as they are absorbed. These carriers cool rapidly to form the population inversion characteristic of lasing. The major disadvantage for this type of laser is that it requires an electron beam, and is consequently expensive and difficult to mass produce. They have been used to make a high brightness replacement for a cathode ray (television) tube. Many types of semiconductors have been tested for this type of lasing, but their development has not been pursued outside of Russia.
Ragle et al., U.S. Pat. No. 4,891,815 (1990), proposed a laser that would be made from high gain GaAs photoconductive switches (PCSS). Ragle was based on the information about GaAs PCSS that was available at the time of the patent: total current and device size. From these, an average carrier density was calculated, assuming uniform carrier distribution through the active region of the device. The patent claimed that a laser could be made from this carrier density if a cavity was built around the device, and proposed several examples of how this might be done. However, more recent experiments have shown that high gain PCSS, which produce sufficient carrier densities to lase, always form current filaments. The carrier density is not uniform, but concentrated in tight filamentary bundles. Furthermore, if the PCSS is triggered with uniform illumination, as proposed in the patent, the filaments form like microscopic lightning bolts in jagged, non-linear streaks across the device which are different each time the device is pulsed. Finally, Ragle proposed to make a laser beam that is perpendicular to the current flow. The intersection of such a cavity with the current filaments would be too small to make a laser. Most of the cavity would consist of strongly absorbing GaAs where there are very few carriers, and the threshold to lasing would not reached. Recent research has shown that these conditions do not produce a laser or reproducible light source.
Accordingly, there is a need for semiconductor light sources that can: (1) produce higher peak power than conventional semiconductor lasers (2) are capable of producing high quality coherent beams; and (3) do not require optical or e-beam pumping.
The present invention provides a new type of semiconductor light source that can produce a high peak power output and is not injection, e-beam, or optically pumped. The present invention is capable of producing high quality coherent or incoherent optical emission. The present invention is based on current filaments. The current filaments can form with optical stimulation and high electric fields in certain types of semiconductors. As the charge carriers in the filaments recombine, they can emit visible and near-visible radiation. This emission can either be used directly to produce incoherent light sources or, if the filaments are incorporated into an optical cavity that supplies the appropriate optical feedback, amplification of the emission from the filaments will produce coherent optical emission (lasers).
The present invention provides a light source formed by the electron-hole plasma inside a current filament. The electron-hole plasma can be several hundred microns in diameter and several centimeters long. A current filament can be initiated optically, with an e-beam, or self-triggered, and is pumped by the electric field across a large insulating region. A current filament can be produced in high gain photoconductive semiconductor switches described in xe2x80x9cHigh-power Optically Activated Solid-State Switchesxe2x80x9d, Rosen and Zutavem editors, Artech house (1994), incorporated herein by reference. The new type of light source provided by the present invention has a potentially large volume and therefore a potentially large energy per pulse or peak power available as a single (coherent) semiconductor laser. Like other semiconductor light sources, these light sources will emit radiation at the wavelength corresponding to the bandgap energy (for GaAs 875 nm or near infra red). Immediate potential applications of the present invention include high energy, short pulse, compact, low cost light sources.
A light source according to the present invention comprises a bulk insulating direct bandgap semiconductor substrate and electrical contacts. An electric source is connected across two contacts on the device to produce an electric field in the substrate. An initiator, for example a source of a narrow straight line light beam, impinges on the surface of the substrate and initiates a current filament. Recombination of electron-hole pairs in the current filament causes optical emission. For an incoherent light source, the emission from the filaments can be collected and directed to the field of illumination. For a coherent light source, a laser cavity can be made by reflection of on-axis photons from the ends of the substrate which stimulates additional recombination and emission and can produce a narrow frequency band, coherent beam of light.
Advantages and novel features will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.